An induction heater induces eddy currents and heats an object to be heated which is a load, such as a cooking pan, using a high-frequency magnetic field generated by a heating coil. This induction heater is a focus of attention because of its advantages of high heat efficiency, safety, and cleanliness. Induction heater that can heat objects with low magnetic permeability and high electrical conductivity, such as aluminum and copper, have recently been developed in addition to apparatuses for objects with high magnetic permeability, such as iron, and those with low magnetic permeability and low electrical conductivity, such as nonmagnetic stainless steel.
In these induction heating apparatuses, stray capacitance (equivalent capacity) exists between the heating coil and the object to be heated. If the user touches the object to be heated, a current travels from the heating coil to the ground through the stray capacitance and internal resistance (equivalent resistance) of the user's body. To heat an object with low magnetic permeability and high electrical conductivity, the number of coil windings in the heating coil needs to be greater and the voltage applied to the heating coil needs to be higher than when heating an object with high magnetic permeability or an object with low magnetic permeability and low electrical conductivity. This may cause leakage of current exceeding a predetermined level from the high-voltage heating coil to the human body. The passing of a leak current through the human body thus needs to be prevented in the case of an induction heater designed to heat objects with low magnetic permeability and high electrical conductivity. For example, the Japanese Utility Model Unexamined Publication No. 50-82046 provides a conductive film on the rear face of a top plate, and this conductive film is grounded so as to prevent any leak current from passing through the human body.
The induction heater adjusts the distribution of heating temperature in the object to be heated by changing the content of the magnetic flux reaching the object. For example, Japanese Patent Unexamined Publication No. 7-249480 provides a ring-shaped electrical conductor between the heating coil and the object to be heated for adjusting the temperature distribution. In this case, the electrical conductor has a slit between the outer periphery and inner periphery. An induction current in a direction opposite the high-frequency current of the heating coil flows in the electrical conductor, but the slit shuts off this induction current. The temperature distribution in the object to be heated is adjusted by regulating the distribution of intensity of magnetic field using the high-frequency current flowing in the heating coil and the induction current flowing in the electrical conductor.
For heating an object with low magnetic permeability and high electrical conductivity, the ring-shaped electrical conductor disposed between the heating coil and the object reduces the buoyancy exerted on the heated object. In this case, however, the heating value of the electrical conductor increases. Accordingly, in the case of apparatuses with a top control panel, which are becoming increasingly popular, the electrical conductor needs to be provided in a position distant from the top control panel to prevent any damage by the heat from the electrical conductor to the liquid crystal device (LCD) provided below the top plate for top-panel operation. In addition, a compact design in both width and height is required by high-density assembly inside the casing to satisfy demand for higher performance.
However, in a conventional structure, the conductive film for preventing the leak current from flowing to the human body and the electrical conductor for decreasing the buoyancy, when heating an object with low magnetic permeability and high electrical conductivity, are separate components. Accordingly, costs are high for an induction heater which has these functions. In addition, more compact designs are demanded.